In current direct printing processes, such as ink jet direct printing, an important print process parameter is the print head to media gap. To accomplish direct-to-paper printing, the paper media has to be carefully and accurately registered, and held down so that it does not come in contact with the print heads. Media gaps may be on the order of 0.5 mm in order to minimize the pixel placement errors due to misdirected jets. Such tight print head to media gaps pose a serious challenge for any cut sheet printer, since the sheet lead edge (LE) and trail edge (TE), and to a lesser extent the sheet body, do not lie perfectly flat. Small departures (<0.1 mm) in local flatness may induce a pixel placement error that may cause an image quality defect. Larger departures (>0.5 mm) in local flatness can cause contact between media and the print head front face. This is undesirable since media particles could be forced into nozzles and any anti-wetting coating on the front face may be damaged. For accurate pixel placement and color registration, it is desirable to keep the print head to media gap within a +/−0.1 mm range about the nominal. However, in order to avoid print head front face damage, the media should not be permitted to close the gap and contact the print head.
Currently known paper hold-down technologies include; “mechanical grippers”, “electrostatics”, “vacuum” and combinations of these systems and devices. Gripper systems can reliably hold sheet edges down, however these are complex, expensive devices and issues exist if different length media are to be transported. Vacuum sheet transport belts may be used to hold down sheets. However, such transports require a relatively high level of vacuum in order to hold the sheet of media flat, and generating and supplying this level of vacuum adds a significant expense. High levels of vacuum also pull the belt and sheet onto a plate below the belt thereby creating a significant amount of drag on the belt. This slows the belt and increases the wear on tear on the system.
A vacuum system may be supplemented with a sheet pre-curling subsystem which biases the sheets into a downcurl mode, i.e., the LE and TE are curved downwardly. This approach offers little hold down latitude for a sheet having any local upcurl at a corner or side edge. In addition, vacuum systems tend to have leakage at the edges; and therefore, the edges may not be held down in a satisfactory manner. An improvement is to provide higher vacuum pressure along sheet edges, such as the inboard and outboard sheet edges, in order to provide increased hold down force locally along the edges. However, considerable complexity and cost must be added to adapt the vacuum belt transport systems having such locally higher pressures so that they can accommodate media having varying widths.
As an alternative to vacuum hold down systems, electrostatic systems have been employed to hold down media as it passes past a print head. However, the use of an electrostatic charge to hold down sheets has heretofore had limited applications Inks used in many printing processes are capable of being electrically charged. Accordingly, if the electrostatic hold down charge were to cover the printing zone of the sheet, a net electrical charge may be induced in the ink droplets, and the droplets can be deflected by the electric field within the printing zone. Such interaction between the ink and the hold down charge can seriously degrade the quality of the printed image. Accordingly, printing systems using electrostatic hold down are limited to use very low conductivity inks or to apply low net tacking charge on the media, resulting in low sheet tack pressure.
Accordingly, it would be desirable to provide a sheet transport apparatus which is capable of holding sheets in a flat orientation without adding undue cost and complexity.